It has already been proposed to scan a side margin by means of a light beam and to detect the light reflected in order to define a square wave pulse whose edges are differentiated between in order to define the edges of the margin. The accuracy of such a method is relatively low. Furthermore, this method does not enable parallelism to be checked.
It has also been proposed to check the width of the margin by means of optic fibres, the measurement consisting of counting the number of optic fibres illuminated and of comparing this number with a reference value. However, the power of resolution of such a measuring apparatus is relatively low on account of the dimension of the optic fibres. The measurement is thus inaccurate. Furthermore, it is necessary to have one detector per optic fibre. Finally, parallelism is not checked.